Supporters of the Iran nuclear talks expressed frustration with a Senate bill that lets Congress weigh in on a final accord — and disappointment with President Barack Obama for indicating he will sign it.

At best, the legislation will give Tehran more leverage in the talks, they argue. At worst, although far less likely, it could scuttle the negotiations completely.

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“We are disappointed and see this as an unwise compromise with some senators that could compromise prospects for [a] negotiated settlement with Iran,” said Kate Gould, a lobbyist with the Friends Committee on National Legislation, a Quaker organization.

Such concerns are by no means universal. Some backers of the bill, which sailed through a Senate committee Tuesday afternoon on a 19-0 vote, dismiss them as over-hyped and say the president’s willingness to compromise suggests he thinks he can ultimately sell the Iran deal on the Hill.

Still, the reactions underscore the intensity of the conversations around the legislation and the Iran negotiations as a whole. Some of the groups opposed to the bill already are trying to build opposition in the House, where they hope to thwart the bill or at least assure it won’t be veto-proof.

The bill was hammered out by Senate Republicans and Democrats despite a presidential veto threat and numerous attempts by Obama administration officials to convince lawmakers to hold off. The version that passed Tuesday eliminated or watered down some provisions that had been especially troublesome to the Obama administration, and the White House emphasized that the president would still veto the bill if unpalatable amendments are tacked on later.

The compromise version of the bill prevents Obama from waiving congressional sanctions on Iran for at least 30 days after a final deal is reached with Iran. Then, Congress can choose to disapprove of the deal by stopping Obama from lifting the sanctions — a resolution that could also be subject to a presidential veto.

Activists on both sides of the debate generally agreed that international negotiations with Iran are likely to move forward, regardless of what Congress does. The U.S. and its partners unveiled a preliminary framework agreement earlier this month in Lausanne, Switzerland, and it appears all sides feel too much is at stake to stop talking ahead of a June 30 deadline.

However, as key details are hammered out, the specter of congressional oversight could change the dynamics in the negotiating room, said Jamal Abdi, policy director for the National Iranian American Council.

“It’s going to make it a lot more difficult to get to a deal,” Abdi said. “It creates a bargaining chip for the Iranians. It isolates the U.S. from its negotiating partners.”

He and others noted that Iran’s parliament may also decide it wants to weigh in on the deal, although in the Islamic Republic, the person with the ultimate say is the theocracy’s top figure, Supreme Leader Ayatollah Ali Khamenei.

The bill must pass votes in the full Senate and House before it can go to Obama for his signature.

Despite the president’s acceptance of the current version, supporters of the talks say they are deeply troubled by some of its core elements, including preventing the president from waiving sanctions on Iran for least 30 days and effectively giving Congress the ability to disapprove of the final deal.

The delay imposed on the president’s ability to waive sanctions could lead Iran to hold off on actions it is required to take under the agreement, some opponents of the bill argued.

To be sure, the president could in the end veto a congressional attempt to kill the deal by stopping him from lifting sanctions. And it’s unlikely enough Democrats would be willing to join Republicans to override that veto.

Still, the prospect that it could reach that stage injects more risk into the ongoing talks, which could embarrass the U.S. on the international stage and open the door to endless domestic debate on the issue, activists argued.

“I’m skeptical of the notion that the bill will provide a managed process for Congress to deal with an Iran deal, rather than a free-for-all like what we have seen with Obamacare votes,” Abdi said. “If the administration can only win by vetoing a resolution of disapproval, rather than soundly defeating it through regular procedure, I think there will be many more battles beyond the” Senate bill.

Daryl Kimball, executive director of the Arms Control Association, said that ultimately, “the bill and the process is politically motivated congressional sausage-making, not good policy.”

Supporters of congressional oversight brushed off those concerns, saying that if Obama can’t corral enough Democrats to support the bill in the long run, that’s a sign there are serious issues with it. If anything, Obama coming to terms with Congress now could make the agreement stronger in the long run, some said.

“I think if the president can persuade the Congress to support his negotiating position he can cement the deal in a sort of a legislative consensus and a bipartisan consensus that makes this deal much more enduring,” said Ray Takeyh, a senior fellow at the Council on Foreign Relations who has been skeptical of the nuclear talks.

Takeyh was one of a group of two dozen Iran nuclear experts and others who signed on to a statement released Monday that praises efforts to give Congress oversight of the deal.

The U.S.’s partners in the negotiations with Iran are Russia, China, Britain, France and Germany. European diplomats contacted Tuesday said their nations are watching the moves in Congress carefully.

One diplomat noted that there’s little debate in Europe on the merits of the negotiations with Iran or the framework agreement so far. “What we as the Western countries in those negotiations want to keep in mind is that opponents in Iran are already trying to build a narrative that the West cannot be trusted and cannot deliver. That is an impression that we would want to avoid,” the diplomat said.

Although there may be enough senators willing to override a presidential veto on the bill, its fate in the House is less certain. Groups such as the Friends Committee said they are starting to focus more on shoring up support for the nuclear deal among House members.

Ilan Goldenberg, director of the Middle East Security Program at the Center for a New American Security, described himself as being agnostic on the congressional oversight bill.

The fact that the president is apparently resigned to the Senate version of the bill suggests that the White House does not think it poses an overall threat to the nuclear agreement, he said. As far as the international talks go, both sides have too much to lose to give up now, no matter what Congress does, he said.

And although the Iranians might get spooked, at the end of the day, they are “pretty sophisticated or at least their negotiators are” about U.S. politics.